Stainless steel and method



United States Patent 3,253,908 STAINLESS STEEL AND METHOD Harry Tanczyn, Baltimore, Md, assignor to Armco Steel Corporation, a corporation of Ohio No Drawin Filed Nov. 20, 1959, Ser. No. 854,226 5 (Ilaims. (Cl. 75--124) This invention relates to the heat-hardenable chromium-nickel-aluminum steels, more particularly the composition of the steel, a method of heat-hardening the same, and the resulting hardened steel itself.

Among the objects of the invention is the provision of a particular chromium-nickel-aluminum stainless steel which is comparatively soft and workable in one condition of heat-treatment and which readily may be hardened for use or service by treatment at temperatures which are not unduly high, to achieve a hard, strong and tough steel, a steel having excellent hardness and strength at room temperatures and at elevated temperatures together with reasonably good ductility and impact strengths, which stainless steel lends itself to hot-working as by forging, rolling and piercing, into a variety of flat, round special shapes for fabrication and heat-treatment to yield a host of articles and products of ultimate use made to the desired specification and tolerances with a minimum loss of metal in treatment.

Other objects of my invention in part will be obvious and in part pointed out in the course of the following specification.

My invention accordingly resides in the combination of elements and relative proportioning of the same, as well as in the various heat-treating steps and the relation of each of the same to one or more of the others, and in the final heat-treated product, all as described herein and particularly set forth in the claims at the end of this specification. V

As conducive to a better understanding of certain features of my invention it may be noted at this point that the stainless steels generally are defined'as those steels which comprise about to 35% chromium, with or without nickel, and with or without additions of any one or more of a number of ingredients employed for special purposes. These ingredients commonly include small amounts of silicon, copper, cobalt, molybdenum, aluminum, tungsten, vanadium, titanium, columbium, and the like. The carbon and manganese contents usually are low, although for special purposes, these may be substantial.

Many grades of stainless steel are now on the market. Of these it generally is considered that it is the chromiumnickel grades which possess the best working and forming characteristics. These steels readily may be hotworked and cold-worked. They may be bent, pressed, stamped, punched and machined to form a host of useful articles, products and component parts. And many of the chromium-nickel stainless steels are greatly benefited through the addition of one or more of the strong carbide-forming elements titanium and columbium. Others are benefited through the addition of copper. And in still others through the addition of aluminum.

The chromium-nickel-aluminum stainless steels presently available not only are comparatively soft, ductile and readily workable in one condition of heat-treatment, but they are easily hardened by heat-treatment to give a steel which is of substantial strength. In many applications, however, it is found that these steels suffer in ductility and impact strength and that greater hardness and higher strength at room temperatures and elevated temperatures are desirable.

One of the objects of my invention, therefore, is to provide a particular chromium-nickel-aluminum stainless steel which possesses the many desirable characteristics of the known chromium-nickel-aluminum stainless steels in matters of workability and formability, as well as subsequent hardenability, together with improved hot-working conditions at the steel mill in addition to possessing improved hardness and strength in heat-hardened condition, both at room temperatures and at elevated temperatures, and in addition possesses improved ductility and impact strength.

In the practice of my invention I provide a chromium nickel-aluminum stainless steel of substantial molybdenum and nitrogen contents and a minimum carbon content. The chromium content of my steel amounts to about 9% to 20%, the nickel content from 2.50% to 8.00%, manganese up to 8.00%, with manganese inversely proportioned with respect to the nickel content, the aluminum from .70% to 2.50%, the molybdenum content 1.0% to 5.0%, with the sum of the chromium and molybdenum contents 14% to 21%, the nitrogen content from .10% to .40%, and the carbon content not exceeding .12%, with remainder substantially all iron. The proportioning of these several ingredients is in every sense critical. Where the chromium content is less than 9% and the sum of the chromium and molybdenum contents less than 14%, the corrosion resistance char-- acteristics seriously suffer. And where the chromium exceeds 20% and the sum of the chromium and molybdenum contents exceeds 21%, I find that inadequate hardening is achieved. The molybdenum content of the steel is at least 1%, for with less than this both hardness and strength suffer. And with more than 5% it appears that there is a loss of corrosion resistance in certain media. Moreover, no benefit seems to derive from an excess of molybdenum. And it is an expensive addition.

-A nickel content of anything less than 2.50% gives a steel which is objectionably hard in the annealed condition with consequent loss of forming properties. With a nickel content exceeding 8.00% I find that the steel inadequately responds to heat-treatment and fails to develop the desired hardness in the final heat-hardened condition.

Manganese, of course, is present, this in amounts up to 2.00% as a maximum. Where desired, however, the nickel content of the steel may be partially replaced by manganese on a 2 for 1 basis, that is, 2% manganese for every 1% of nickel replaced. But I find that there must be an actual nickel content of at least 2.50%, and that the total manganese content, this including the amounts commonly present and the amounts employed for replacing the nickel, shall not exceed 8%. For with an excessive amount of manganese there is a loss of ductility and impact strength in the final heat-hardened condition.

The aluminum content is critical, too, for with an aluminum content less than .70% inadequate hardening is had and where it exceeds 2.50% the steel suffers a loss of hardenability, does not harden to the full extent.

In my steel the carbon content should not exceed .12% because with a higher carbon content I find a definite loss of corrosion resistance, especially in pickling the steel as with the known nitric acid-hydrofluoric acid pickling solutions commonly used to remove heat-tint from the heat-treating operations referred to hereinafter. Actually, I much prefer that the carbon content shall not exceed 0.08% in order to positively assure freedo from corrosive attack.

Both molybdenum and nitrogen are essential ingredients as noted above. For I find that both lend hardness, to the steel in final heat-treated condition. And nitrogen contributes to ductility and impact strength. Where there is employed nitrogen in amount less than .10%, the high ductility and impact strength of my steel is lost. And

where it substantially exceeds .40% the composition balance is adversely affected; the steel is inclined to become objectionably hard in the annealed condition with consequent loss of working and forming properties.

The silicon content of my steel amounts to about 2.00% as a maximum, the phosphorus content a maximum of 050%, and the sulphur content a maximum of 050%.

The steel of my invention therefore essentially consists of chromium in the amount of 9% to 20%, nickel in the amount of 2.50% to 8.00%, manganese a maximum of 8.00% with manganese 8.00% when nickel is 2.5% and 2% maximum where nickel is 8.00%, aluminum in the amount of .70% to 2.00%, molybdenum in amount of 1% to 5%, with the-sum of the chromium and molybdenum contents 14% to 21%, nitrogen in the amount of .10% to .40%, carbon .12% maximum, preferably 08% maximum, and more particularly a maximum of .O3%, with silicon 2.0% maximum, phosphorus and sulphur each 050% maximum, with the remainder of the steel substantially all iron. The preferred steel of my invention as appears more fully hereinafter analyzes 15.0% to 16.0% chromium, 6.0% to 7.5% nickel, .8% to 1.5% aluminum, 2.0% to 3.0% molybdenum, .10% to .20% nitrogen, carbon 08% maximum, manganese 1.5% maximum, silicon 1.0% maximum, with the remainder substantially all iron.

My steel is melted in accordance with the procedures generally described and claimed in one or more of the US. Letters Patent 1,925,182 of September 5, 1933, issued to Alexanrer L. Feild, and entitled, Process for the Manufacture of Rustless Iron, Patent 2,455,073, issued November 30, 1948, to Donald L. Loveless and entitled, Production of Stainless Steel, and Patent 2,621,119 of December 9, 1952, issued to Donald L. Loveless and entitled, Stainless Steel Melting Process. The required nitrogen content is conveniently introduced in the manner described in the Weitzenkorn US. Patent 2,454,020 of November 16, 1948, entitled, Ferrochrome Process and Product. For the steels of high manganese, nitrogen is introduced in accordance with the method described in US. Patent 2,696,433 of December 9, 1954, entitled, Production of High Nitrogen-Manganese Steel.

The steel, obtained in the form of ingots, is fashioned into blooms and billets and thence converted into plate, sheet, strip, bars, rods, wire and tubes at the mill. 1 find that it possesses improved hot-working characteristics, this as compared with the prior chromium-nickel-aluminum stainless steels.

The various converted products are annealed at a temperature of 1800 to 2100 F. and either cooled in air or quenched in water, asdesired. The time at annealing temperature does not appear to be particularly critical; usually one-half hour at temperature is sufficient. The steel in the annealed condition is substantially fully austenitic, any free-ferrite not exceeding about 10% by volume. The aluminum content of the steel appears to remain in solution even after quenching or cooling. The metal is reasonably soft and ductile. The hardness does not exceed Rockwell B99. It is readily worked and formed as by bending, drawing, spinning. And it is easily machined. Moreover, it may be welded, by known methods such as the electric are or the gas torch. With fabrication there are had a variety of articles, products and parts for industry and commerce. The steel is readily fashioned into various structural members required for airplanes. It is easily made into bolts, screws and nuts. It is suitably fashioned into valves and valve parts, die blocks and surgical instruments. In the working, forming and fabrication of the steel particular advantage is taken of its good workability and formability in the annealed condition. These various working, forming and fabricating operations, of course, are usually performed by the customer-fabricator.

Following fabrication the various articles, products and parts are heated at a temperature of about 1400 F. for a sufficient length of time to effect a precipitation of the carbides present in the steel, presumably carbides of chromium. There also may be some precipitation of aluminum compounds. And the steel, upon cooling, as by water-quench, transforms into martensite or a martensite-like constituent with some free-ferrite present. The steel in this condition, for a typical example, has a hardness amounting to some Rockwell C28/ 29.

The high molybdenum, high nitrogen chromium-nickelaluminum stainless steel of my invention retains sufiicient softness and ductility in the preliminarily hardened condition to permit working as by cold-rolling and drawing, and machining as by turning, cutting, punching, drilling, and the like. Where desired, therefore, one or more of the various working, forming and fabricating operations referred to above may bedelayed and practiced on the preliminarily hardened steel rather than upon the annealed metal coming from the mill. I

In the further practiceof my invention I find that as a result of the preliminary hardening treatment at a temperature of some 1400 F. there is formed on the various stainless steel articles, products and parts a light heattint. It is conveniently removed by pickling in an aqueous solution of nitric and hydrofluoric acids. In general, the nitric acid content amounts to about 15 by volume and the hydrofluoric acid about 2%, with remainder water.

My steel is possessed of superior corrosion resisting properties; even the thinnest sections, that is, sections on the order of .005" to .010", are reliably pickled without fear of metal loss or penetration through the metal as occasionally encountered in the steels of the prior art. My steel in no way suffers from intergranular attack. This I attribute to the extremely low carbon content, with the consequent formation of a bare minimum of objectionable carbides.

The preliminary hardened chromium-nickel-aluminum stainless steels of high molybdenum and nitrogen contents and low carbon content are now given a final hardening treatment by reheating at a temperature of about 1050" F. for about an hour. While the temperature may range from some 750,to 1100 F. for a time of half an hour at the higher temperature to some 2 hours or more at the lower temperature, I find that a temperature of 1050 F. gives excellent results. With final hardening treatment and cooling in air or water the martensite or martensite-like constituent of the preliminarily hardened steel is fully retained.

In the final hardening treatment, it is my view that there is precipitated an aluminum compound, very likely a compound of nickel and aluminum, which gives rise to the further hardening. It may be that the hardening reaction involves some rearrangement or reordering of the lattice structure of the aluminum compound within the lattice structure of the austenitic matrix. However all that may be, the steel of my invention, as finally hardened by heattreatment, is hard and strong and yet reasonably ductile and tough. The hardness had amounts to some Rockwell C42 as a minimum.

As specifically illustrative of my invention I have prepared two chromium-nickel-aluminum stainless steels of high molybdenum and nitrogen contents and low carbon content. The chemical analyses of these examples are given in Table I below:

TABLE I.-CHEMICAL ANALYSES OF Cr-Ni-Al STAINLESS STEELS OF HIGH MOLYBDENUM AND NITROGEN CONTENTS, AND LOW CARBON CONTENT Heat No. C Mn P S S1 C1 Ni Al M0 N R440 018 56 008 011 51 15. 03 7. 37 l. 04 2. 53 12 R423 063 55 i 010 007 57 15. 30 7. 30 1. 06 2. 49 15 The two specific'examples of my stainless steel as illusbon .12% maximum, manganese 8.00% maximum, with tratively set forth above were annealed at a temperature manganese inversely proportioned with respect to the of 1900 for a period of one-half hour and then quenched nickel content, silicon 2.00% maximum, phosphorus in water. Following annealing and quenching they were .05 maximum, sulphur 050% maximum, and remainheated at a temperature of 1400" F. for a period of 1 /2 der substantially all iron. hrs. and water-quenched, after which they were reheated 2. A chromium-nickel-aluminum stainless steel suscepat1050 F. for 1 /2 hrs. and cooled in air. The hardness tible to precipitation-hardening by double heat-treatment had in these several samples in the various conditions of fro a oft, orkabl au teniti condition to give great annealing, pr limin ry ha d ning and fi al hard n g are strength in combination with good ductility, said steel given in Table 11 below: consisting essentially of 15.0% to 16.0% chromium, 6.0% TABLE II.HARDNESS VALUES OF THE TWO STEELS'OF to nickel, t0 aluminum, to

TABLE I IN THE ANNEALED, PRELIMINARILY HARD molybdenum, .10% to .20% nitrogen, carbon 08% maxi- ENED AND FINAL HARDENED CONDITION mum, manganese 2% maxlrnum, slllcon 1.00% maxlmum,

phosphorus 050% maximum, sulphur .050% maximum,

Annealed Pre. Hard. (An- Final Hard. (Pre.

Heat No. (1,000 F., nealed plus Hard. plus 1,050 F., 15 and remainder substantially all iron.

0 c o I V hr. W.Q.) 1 3 1 E- and All) 3. A chromlum-nlckel-alumlnum stalnless steel susceptible to precipitation-hardening by double heat-treatment R440 Rockwell B99 Rockwell 027.-- Rockwell 043. from a 9 wolkabler alflstenitic condltlon give great R423 Rockwell B94.-- Rockwell o2s Rockwell ole. strength in combination with good ductlllty, said steel con- 20 sisting essentially of 15.0% to 16.0% chromium, 6.0% to 7.5% nickel, .8% to 1.5% aluminum, 2.0% to 3.0% molybdenum, .12% to .15 nitrogen, carbon .03% maxi- The mechanical properties of the two illustrative steels are given below in Table III:

U.'l.S., 0.2% Y.S., Percent Percent Izod- Rockwell Heat N0. p.s.i. p.s.i. E1. in 2 Red. Area Impact Hardness (Ft./Lbs.)

It will be seen that I provide in my invention a chromimu ganese 2% maximum, silicon 1.00% maximum, um-nickel-aluminum steel in which there are had the varip sphor 0% max nu S phur 050% maXlmurn, ous objects hereinbefore set forth, together with many balance lr practical advantages. The chromium-mickel-aluminurn 4. A chromlum-nlckel-alumlnum stalnless steel preclplstainless steel of my invention of high molybdenum and latlon-hardflned y flollble hum-treatment o u nitrogen contents and low carbon content works well in nealed condltlon to give great strength in comblnatlon with good ductility and consisting essentially of 9% to 20% the mill, is of improved corrosion resistance and readily h 2 50 7 8 007 k 1 707 2 507 1 c romium,. oto. oIllC e,. oto. 0 aumilends itself to forming and fabrication by well known methods. These include bending, spinning and drawing, as B 1% l0 5% molybdenum, with the 811m of the well as punching, cutting, drilling and other machining mium and m lybdenum C ntents 14% to-21%, .10% operations. Also these methods include brazing, weldo 0% n t ogen, carbon .12% maximum, manganese ing and soldering. There are achieved a wide variety of maximum, h ma gan s inversely proportioned articles, products and component parts which then are to nickel and .00% Wh n ni kel is 2.5% and 2% maxireadily heat-hardened by double heat-treatment, that is, mum n ni k l i 5 -00%, i i n maximum, a preliminary hardening treatment followed by a final phosphorus 050% maximum, sulphur .050% maximum, hardening treatment. The finally hardened steel is hard and remainder Substantially all ifOIlv and strong at room temperatures and at elevated tempera- 5. A chromium-nickel-aluminum stainless steel preclpitures as well. Moreover, it possesses good ductility and talloll-hafdened y double heat-lfeatmfillt from the toughness, The strength and toughness f my t l li nealed condition to give a hardness of at least Rockwell not only in the direction of rolling or other working but C46, an ultim slrfinglh Of'al least 200,000 P- all in also in the transverse directions of width and thickness. combination with elongation in of at least Said The steel has good corrosion resistance, particularly re- S eel consisting essentially of 15.0% to 16.5% chromium,

sisting intergranular corrosion. 6.0% to 7.5 nickel, .8% to 1.50% aluminum, 2.0% to Since many possible embodiments may be made of 3.0% molybdenum, .10% to 20% nitrogen, carbon 03% my invention and since many changes made in the emmaximum, and remainder substantially all iron.

bodiments herein set forth, it is to be understood that all matter described herein is to be interpreted as illustrative, References Cited y the Examiner alimiiationtg UNITED STATES PATENTS c arm as my lnven ion:

1. A chromium-nickel-aluminum stainless steel suscepa tible to precipitation-hardening by double heat-treatment 25O5763 5 0 l l ers et a 75128' from a soft, workable, austenitic condition to give great 2,868,637 i a strength in combination with good ductlllty, sald steel con- 2:892:702 6/1959 Walton e al- N: 75 124 sisting essentially of 9.00% to 20.00% chromium, 2.50%

to 8.00% nickel, .70% to 2.50% aluminum, 1% to 5% I molybdenum, with the sum of the chromium and molyb- DAVID RECK Examme" denum contents 14% to 21%, .10% to .40% nitrogen, car- RAY K. WINDHAM, MARCUS U. LYONS, Examiners. 

1. A CHROMIUM-NICKEL-ALUMINUM STAINLSS STEEL SUSCEPTIBLE TO PRECIPITATION-HARDENING BY DOUBLE HEAT-TREATMENT FROM A SOFT, WORKABLE, AUSTENITIC CONDITION TO GIVE GREAT STRENGTH IN COMBINATION WITH GOOD DUCTILITY, SAID STEEL CONSISTING ESSENTIALLY OF 9.00% TO 20.00% CHROMIUM, 2.50% TO 8.00% NICKEL, .70% TO 2.50% ALUMINUM, 1% TO 5% MOLYBDENUM, WITH THE SUM OF THE CHROMIUM AND MOLYBDENUM CONTENTS 14% TO 21%, .10% TO .40% NITROGEN, CARBON .12% MAXIMUM, MANGANESE 8.00% MAXIMUM, WITH MANGANESE INVERSELY PROPORTIONED WITH RESPECT TO THE NICKEL CONTENT, SILICON 2.00% MAXIMUM, PHOSPHORUS .050% MAXIMUM, SULPHUR .050% MAXIMUM, AND REMAINDER SUBSTANTIALLY ALL IRON. 